Chapter 4. Mixed robust H ∞ and µ-synthesis controller applied for a reverse
4.2 Membranes
Reverse osmosis membranes have a pore size around 0.0001 µm. The mean size of a water molecule is about 0.097 nm. Hence, water can go through the RO membrane while the other factors with bigger sizes are prevented. After water passes through a reverse osmosis membrane, it is essentially pure water. In addition to removing all organic molecules, bacteria (sizes from 0.2 to 10 µm) and viruses (sizes from 0.02 to 0.4 µm), reverse osmosis also removes most minerals that are presented in the water. Reverse osmosis removes monovalent (eg. NaCl) ions, which means that it desalinates the water.
Fig. 25 RO filtration
4.2.1 Structure and material
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Two materials make up the bulk of commercial RO membranes are cellulose acetate and composite. The composite membranes usually exhibit higher rejection at lower operating pressures than the cellulose acetate. The current RO membrane market is dominated by thin film composite (TFC) polyamide types. This kind of membrane consists of three layers: A polyester web acting as structural support (backing), a microporous interlayer web, and an ultra-thin barrier layer on the upper surface which is 0.2 μm (see Fig. 26). The polyester support web has almost no effect on membrane transport properties. It only has the effect on supporting the membrane’s structure. Between the barrier layer and the support layer, a micro- porous interlayer of polysulfonic polymer is added to enable the ultra-thin barrier layer to withstand high pressure compression. The thickness of the barrier layer is reduced to minimize resistance to the permeate transport. Membrane pore size is normally less than 0.6 nm (0.0006 µm) to achieve salt rejection consistently higher than 99%. The selective barrier layer is often made of aromatic polyamide. With improving chemical resistance and structural robustness, it offers reasonable tolerance to impurities, enhanced durability and easy cleaning characteristics (Lee, 2011)
Fig. 26 The structure of RO membrane
4.2.2 Hollow fine fiber membrane module
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This configuration uses membrane in the form of hollow fibers. These fibers may be extruded from cellulosic or non-cellulosic material, which have the minimum hollows size up to 42 micron (0.0016 inch). One membrane is a bundle of millions of these fibers folded in half. The bundle is packed inside a pressure tube which usually has a length about 120 cm (4 ft). The pressure tube is sealed at both ends to form a sheet-like permeate output and a brine output which prevents the feed stream from bypassing out. A perforated plastic tube in the center of the pressure tube will serve as a feed water distributor. The assembly is called a permeator. The pressurized feed saline water enters the permeator feed end through the center distributor tube, passes through the tube wall, and flows radially around the fiber bundle toward the outer permeator pressure shell. Water permeates through the outside wall of the fibers into the hollow core of fibers, and to the product end of the fiber bundle, and exits through the product connection at one end of the permeator. The left concentrate water is rejected through brine tube in the other end of permeator.
The permeability of a hollow fiber module is low. Therefore, the concentration polarization is also low at the membrane surface, resulting in a non-turbulent or laminar flow regime. Normally, a single hollow fiber permeator can be operated at up to 50-percent recovery and meet the minimum reject flow required to limit the concentration polarization. The hollow fiber unit allows a large membrane area per unit volume of permeator which results in compact structure. Hollow fiber membranes are available for brackish and seawater applications. Due to their compact structures, hollow fiber modules require feed water of lower concentration than the spiral wound module configuration.
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Fig. 27 The construction and flow patterns in a hollow fiber membrane system (Pfafflin, 2015)
4.2.3 Spiral wound membrane module
In spiral wound structure, a flat-sheet of composite membrane is folded in half with the membrane facing inward. A feed spacer is then put in between the folded membrane to form a membrane leaf. This assembly is sealed on three sides with the fourth side left open for permeate to exit. The mesh spacer is to provide space for feed water to flow between the membrane surfaces, and to induce turbulence and reduces concentration polarization. A permeate spacer is added between membrane leaves, forming membrane assemblies. Some of these assemblies are wound around a central plastic tube. This tube is perforated to collect the permeate water from the multiple leaf assemblies. The feed/brine flow through the element is a cross-flow from the feed end to the opposite brine end, running parallel to the membrane surface.
In order to operate at acceptable recoveries, spiral systems are usually staged with three to six membrane elements connected in series in a pressure tube. The brine stream from the first element becomes the feed to the following element, and so on for each element within the pressure tube. The brine stream from the last element
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exits the pressure tube to waste. The permeate water from each element enters the permeate collector tube and exits the vessel as a common permeate stream. A single pressure vessel with four to six membrane elements connected in series can be operated at up to 50-percent recovery under normal design conditions. The brine seal on the element feed end seal carrier prevents the feed/brine stream from bypassing the following element. In comparison to the hollow fiber membrane, the spiral wound membrane working under lower pressure while the recoveries are equal.
Fig. 28 The construction and flow patterns in a spiral wound membrane system (Pfafflin, 2015)